1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device by using an ALD (atomic layer deposition) technique and, more particularly, to a method of forming a gate insulation film on a silicon substrate of a semiconductor device by using an ALD technique.
2. Description of the Related Art
A MISFET (metal-insulator-semiconductor field-effect-transistor) includes a gate electrode which is formed on a silicon substrate via a gate insulation film, and a pair of impurity diffused regions, which are formed adjacent to the gate electrode and in a surface portion of the silicon substrate. For instance, silicon oxide is used for the gate insulation film, and impurity-doped polycrystalline silicon (polysilicon) is used for the gate electrode for the purpose of avoiding reaction with the silicon substrate or the gate insulation film. For a P-channel MISFET, for example, boron is used as the impurities doped in the diffused regions.
There has been a requirement of semiconductor devices including therein MISFETs to reduce the thickness of the gate insulation film so as to achieve lower power dissipation. However, if the gate insulation film made from silicon oxide is reduced in thickness, boron which is doped in the gate electrode may be diffused through the gate insulation film during an activation treatment thereof, to thereby reach the silicon substrate. The boron which has reached the silicon substrate may cause a significant fluctuation of the threshold value of the MISFETs to degrade the characteristics of the MISFETs, which should be avoided.
On the other hand, a method has been implemented, in which nitrogen is contained in the gate insulation film for the purpose of avoiding the diffusion of the boron therethrough toward the silicon substrate, and also reducing the thickness of the gate insulation film. The nitrogen contained in the gate insulation film raises the dielectric constant of the gate insulation film, and restrains the boron from diffusing and penetrating through the gate insulation film toward the silicon substrate.
In the process of forming the gate insulation film containing nitrogen, it is desired to prevent the diffusion of the nitrogen in the vicinity of the interface between the silicon substrate and the gate insulation film. This is because the nitrogen which has reached the vicinity of the interface causes vacancy defects in the silicon substrate, to raise the problem of various characteristic degradations in the MISFETs, such as reduction in the mobility of carriers.
There is also a known technique for forming a gate insulation film having a two layer structure, which is used for the purpose of avoiding the diffusion of nitrogen toward the vicinity of the interface with the silicon substrate. The two-layer structure of the gate insulation film is obtained by forming, on the silicon substrate, a silicon oxide film and a silicon nitride film in this order.
For the process of forming two-layer gate insulation film, in the view point of improving the reliability of the gate insulation film, there has been an attempt to form a silicon nitride layer by using an ALD technique. The ALD technique is such that a target film of a desired thickness is formed by repeatedly depositing a layer of the target material having a thickness of monoatomic level.
FIGS. 7A and 7B show the steps of forming the silicon nitride film by using the ALD technique. In the process for forming the silicon nitride film by using the ALD technique, two steps are alternately and repeatedly carried out, one being the step of depositing a monoatomic silicon layer 23 by using a Si (silicon) source gas, as shown in FIG. 7A, and the other being the step of nitriding the monoatomic silicon layer 23 by using plasma of a N (nitrogen) source gas, as shown in FIG. 7B, to form a silicon nitride layer 24 having an monoatomic-level thickness. These steps iterated for a number of times finally provide a silicon nitride film having a desired film thickness.
In the process of the ALD technique, a film of a desired thickness is formed by repeating deposition of the layer having a monoatomic-level thickness, and is precisely controlled with respect to the film quality. Therefore, it is possible to form a high-quality film which is sufficient in controllability of the thickness and composition of the film as well as in the in-plane uniformity thereof. Further, since the deposition can be performed at a relatively low process temperature, the range of variation in the impurity profile of the film can be suppressed. Patent Publication JP-2004-006455A, for example, describes a fabrication method of a semiconductor device including the step of forming a silicon nitride film by using the ALD technique, in order to form a two-layer gate insulation film configured by a silicon oxide film and the silicon nitride film.
It is known in the process of depositing the silicon nitride film by using the ALD technique that a plurality of island silicon nitride layers 24a are first formed, as shown in FIG. 8, and then the island silicon nitride layers 24a are eventually allowed to couple together and form the continuous silicon nitride layer 24, as shown in FIG. 7B. In the process step shown in FIG. 8, the silicon oxide film 13, which is in general likely to diffuse nitrogen therethrough, is exposed to the ambient and thus directly nitrided. Therefore, there occurs the problem that a large quantity of nitrogen diffuses through the silicon oxide film 13, reaching the vicinity of the interface between the silicon substrate 11 and the silicon oxide film 13.
In order to suppress the diffusion of the nitrogen toward the vicinity of the interface between the silicon substrate 11 and the silicon oxide film 13, it may be considered to use a lower process temperature during the nitriding of the silicon oxide film 13, a lower flow rate of the N-source gas and/or a smaller time length for exposure of the wafer to the N-source gas in the nitriding step, to thereby suppress nitriding of the underlying silicon oxide film. However, in any of those known countermeasures, another problem is caused that a sufficient amount of nitrogen is not supplied in the silicon nitride film, thereby weakening the Si—N bonding strength therein, which causes an insufficient ability of the film for suppressing the diffusion of boron therethrough.